The defibration of different wood materials is facilitated if the fiber material is first preheated with steam (i.e. water vapor) for a certain period of time, preferably at increased pressure and increased temperature. The so-called thermomechanical pulping (TMP) process is designed in this way and produces pulps with a high yield (.gtoreq.95%). The brightness of such pulps is affected by the type and quality of the wood material used, but usually is 55-60% (ISO).
An addition of chemicals before or during the mechanical treatment has been found to result in improved strength properties of the pulp compared to the properties of a purely thermomechanical pulp. Chemithermomechanical pulps (CTMP, yield usually &lt;95%) are produced today, but then at chemical batching levels which considerably exceed those used in the present invention. The results published so far show that with such a high and increased concentration of chemicals, an increased tensile strength of the paper is obtained at constant drainability (expressed as millilitres Canadian Standard Freeness), but at the expense of an increased total expenditure of beating energy. Another adverse effect of an increased addition of chemicals has been reported, viz. a pronounced decrease in the light-scattering ability (opacity). Besides certain strength properties, a high light-scattering coefficient is an essential requirement in connection with printing paper pulps and, therefore, the chemical strength-improving treatment had to be limited so that the optical properties will not deteriorate too much.
When producing fiber pulps it is known (Ford et al, U.S. Pat. No. 4,116,758, Sept. 26, 1978) to expose the starting materials to a combined chemical, thermal and mechanical treatment to increase the strength properties of the finished pulp. In said patent it is stated that, to be able to obtain a sufficiently high strength but still maintain an acceptable yield, when treating spruce wood, it is necessary to add fairly large amounts of chemicals. Thus, as a lower limit for the added amount of chemicals, the patentees state that it is necessary to add at least 85% and preferably at least 90% of the maximum level of sulfonation which for spruce is stated to be 0.65 wt % S, as combined sulfur. Thus, when treating spruce in the manner taught by Ford et al, it is necessary to add an amount of chemicals giving at least 0.55 wt % chemically bonded sulfur. The upper limit of the added amount of chemicals is determined by the objective of obtaining at least 90 wt % yield. The high added amounts of chemicals in this prior art method make it necessary to use a separate sulfite recovery system in view of the economy and the present environmental demands.
As examples of other prior art processes in which a fiber pulp is produced by chemical and mechanical treatments of wood fibers mention may be made of British Pat. GB-A No. 1,546,877 and Australian Pat. No. 469,905. Thus, GB-A No. 1,546,877 describes a process of producing a light, lignin-rich, absorbing pulp at a high yield to be used as a raw material for producing tissue, cellulosic wadding, diapers, sanitary towels and tampons. In this process, the objective is to obtain a pulp with high absorbancy at a high yield and no mention is made of the strength of a paper made from the pulp produced. The Australian Pat. No. 469,905 describes a process of treating wood fibers using a solution of sodium hexametaphosphate, possibly in combination with sodium sulphite. However, high amounts of chemicals are used according to this patent.